Wire grid polarizers are made by fabricating a very fine grid pattern of metal stripes on an outer surface of a thin substrate, typically glass. The fabrication techniques generally use lithographic exposures that are best completed on wafers that can be easily handled by semiconductor fabrication equipment.
When used as a polarized light beam splitter (PBS) control element, the wire grid plate is tilted at a forty five degree angle with respect to a light source to reflect S-polarized light, for example, onto a mirror and a ¼ waveplate, or onto a reflective liquid crystal display (LCD) device. The desired output is modulated to P-polarization and then reflects through the tilted wire grid polarizer. This tilted plate configuration introduces two problems. First, the tilted glass substrate will introduce astigmatism into the projected image. This will cause horizontal and vertical features of the desired projected image to come to focus in separate image planes. The second problem is that the tilted plate occupies a certain space and the projection optics must have sufficient back working distance to operate over the length of that space. This can be a problem if the desired focal length of the projection lens is small.
Most PBS structures use a cubic prism configuration that imbeds the polarizing surface inside a cube of glass. This cubic configuration presents flat sides perpendicular to the optical axis of the projection system which introduces no astigmatism into the projected image. Additionally, because the cube is made of glass having an index of refraction greater than air, the effective back working distance of the projection optics will be reduced.
Wire grid polarizing structures are best manufactured on flat, thin substrates. Currently, there is no convenient method of producing the grids directly on a prism face. Additionally, operation of the wire grid requires air on the metal side of the grid, which prevents the wire grid from being glued or abutted directly against a glass prism face. Due to these manufacturing and operational limitations, there appears to be no prior art solution to fabricate a viable PBS prism using an embedded wire grid polarizer.